1. Field of the Invention
The present invention relates to a method of manufacturing a substrate for a photovoltaic cell, and more particularly, to a method of manufacturing a substrate for a photovoltaic cell, in which the high optical characteristic in a long-wavelength range available for the photovoltaic cell can be maintained, and at the same time, the amount of hazing can be increased.
2. Description of Related Art
Recently, as a measure against the shortage of energy resources and environmental pollution, the development of photovoltaic cells is underway on a large scale. A photovoltaic cell is a key device for photovoltaic power generation that directly converts solar energy into electric energy. Photovoltaic cells are currently being applied in a variety of fields, including the supply of electricity to electrical and electronic products, houses and buildings as well as industrial power generation. The most basic structure of such photovoltaic cells is a P-N junction diode. Photovoltaic cells are divided into a variety of types, for example, a silicon (Si) photovoltaic cell, a chemical photovoltaic cell, a dye-sensitized photovoltaic cell, and a tandem photovoltaic cell, depending on the material used in the light-absorbing layer. Specifically, the Si photovoltaic cell uses Si for the light-absorbing layer, and the chemical photovoltaic cell uses CuInSe2 (CIS) or cadmium telluride (CdTe) for the light-absorbing layer. In the dye-sensitized photovoltaic cell, photosensitive dye molecules are adsorbed on the surface of nano particles of a porous film, and electrons are activated when the photosensitive dye molecules absorb visible light. The tandem photovoltaic cell has a plurality of amorphous Si layers stacked on one another. In addition, photovoltaic cells are also divided into bulk type photovoltaic cells (including single crystalline and polycrystalline types) and thin film type photovoltaic cells (including amorphous and microcrystalline types).
At present, bulk type photovoltaic cells using polycrystalline Si occupy 90% or more of the entire market. However, the bulk type crystalline Si photovoltaic cell has a problem in that the unit cost for photovoltaic power generation is expensive, i.e. 3 to 10 times as great as the cost of thermal power generation, atomic power generation, hydraulic power generation, or the like. This is because the bulk type crystalline Si photovoltaic cell uses a large amount of Si as a raw material, which is expensive, and because the complicated manufacturing process increases the cost of manufacture.
In order to solve such problems, recently, thin film photovoltaic cells using amorphous Si (a-Si: H) and microcrystalline Si (mc-Si: H) are being developed and commercially distributed.
In the meantime, a transparent conductive film (typically, a TCO film) used in photovoltaic cells is required to exhibit high light transmittance, high conductivity, and a high light-trapping effect. In particular, a tandem type thin film photovoltaic cell (see FIG. 7) is required to have high light transmittance and a high haze value in a wide wavelength band ranging from 350 nm to 1200 nm. In addition, when the light-absorbing layer is formed in the transparent conductive film, resistance to hydrogen plasma is also required.
The transparent conductive film that is most widely used for photovoltaic cells at present is a conductive film that contains tin oxide (SnO2) as the major component. However, the SnO2 transparent conductive film has limitations due to its material properties. That is, it exhibits a low light transmittance in a long-wavelength range of 900 nm or more, which leads to low photoelectric transformation efficiency of photovoltaic cells. It also has a low electrical conductivity and a low transmittance of about 70%. In addition, the SnO2 transparent conductive film suffers from an increase in resistance and a decrease in transmission characteristic because its surface is damaged by the formation of a large amount of hydrogen plasma when p-type Si is deposited during the processing of photovoltaic cells.
In addition, indium tin oxide (ITO), which is used for the transparent conductive film together with SnO2, satisfies the requirements for high light transmittance of 80% or higher and an excellent electrical conductivity of 10−4 Ωcm. However, ITO also has problems in that the price of the rare raw material In, which is used as the major component, is continuously increasing, and in that In exhibits a high reduction rate in the hydrogen plasma processing and resultant chemical instability.
In order to solve such problems, the development of a transparent conductive film that can replace the transparent conductive films having SnO2 or In as the major component thereof is underway. Recently, there has been strong interest in zinc oxide (ZnO) as the ideal material. ZnO has an advantage in that its electro-optical properties are readily adjustable depending on the doping material, since it can be easily added and has a narrow conduction band. In addition, a transparent conductive film having ZnO as the major component is stable in hydrogen plasma processing, is manufactured at a low cost, and has high light transmittance.
Such a ZnO transparent conductive film is manufactured by chemical vapor deposition (CVD). However, the surface of the ZnO transparent conductive film, which is manufactured by CVD, is configured such that it has very sharp grooves. This sharp groove structure, however, acts as a source for defects during the deposition of amorphous Si, ultimately becoming a cause of decreased efficiency of the photovoltaic cell to which the ZnO transparent conductive film is applied.
The transparent conductive film used as the transparent electrode of a photovoltaic cell is required to exhibit high transmittance, high conductivity, and a high haze value. However, the ZnO transparent conductive film has the problem of structural defects caused by the groove structure in the surface, as mentioned above, despite the increase in hazing attributable to the groove structure.
The information disclosed in this Background of the Invention section is only for the enhancement of understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.